1. Field of the Invention
Methods consistent with the present invention relate to hopping frequencies of an orthogonal-frequency-division-multiplexing (OFDM) symbol, and more particularly, to mapping a method of grouping sub-carriers and setting of positions of pilots with reference to reserved bits in a physical (PHY) layer header before transmitting data using an OFDM modulation method so as to hop frequencies of an OFDM symbol.
2. Description of the Related Art
In general, an OFDM system transforms symbols input in series into parallel symbols having predetermined sizes, multiplex the parallel symbols into orthogonal sub-carrier frequencies, and transmit the parallel symbols over the orthogonal sub-carrier frequencies.
A multi-band (MB)-OFDM method is characterized in that a plurality of frequency bands of an OFDM symbol is hopped in a unit of a symbol to transmit a signal. For example, the MB-OFDM method is modulation technology used in a specific wireless communication system such as an ultra wide band (UWB) system. OFDM modulation technology and frequency hopping technology are combined into MB-OFDM modulation technology.
An MB-OFDM system divides a predetermined frequency band into a plurality of sub-bands. The MB-OFDM system can transmit data (a symbol) using the plurality of sub-bands so as to transmit or receive a large amount of data per unit time. A UWB system selects one of the plurality of sub-bands and uses the selected sub-band according to set regulations so as to improve security of data.
FIG. 1A is a view illustrating a plurality of sub-bands to be used in an MB-OFDM system. As shown in FIG. 1A, a center frequency of a frequency band of the MB-OFDM system ranges from 3432 MHz to 10296 MHz. The frequency band of the MB-OFDM system is broadly divided into five groups. Each of first through fourth groups of the five groups includes three sub-bands, and a fifth group includes two sub-bands.
Center frequencies of the three sub-bands of the first group are 3432 MHz, 3960 MHz, and 4488 MHz, respectively, and center frequencies of the three sub-bands of the second group are 5016 MHz, 5544 MHz, and 6072 MHz, respectively. Center frequencies of the three sub-bands of the third group are 6600 MHz, 7128 MHz, and 7656 MHz, respectively, and center frequencies of the three sub-bands of the fourth group are 8184 MHz, 8712 MHz, and 9240 MHz, respectively. Center frequencies of the two sub-bands of the fifth group are 9768 MHz and 10296 MHz, respectively.
Table 1 below shows a method of transmitting payloads according to a transmission rate in the MB-OFDM system.
TABLE 1TransmissionModulationEncodingSpreadingRateMethodRateConjugateTSFGain53.3QPSK⅓∘2480QPSK½∘24106.67QPSK⅓x22160QPSK½x22200QPSK⅝x22320DCM½x11400DCM⅝x11480DCM¾x11
In a case where the transmission rate ranges from 53.3 Mbps to 200 Mbps, the MB-OFDM system uses a quadrature phase shift keying (QPSK) method. In a case where the transmission rate ranges from 320 Mbps to 480 Mbps, the MB-OFDM system uses a dual carrier modulation (DCM) method.
In a case where the transmission rate ranges from 53.3 Mbps to 80 Mbps, the MB-OFDM system transmits a conjugate symbol. Thus, the spreading gain is “4.” In other words, in a case where the transmission rate ranges from 53.3 Mbps to 80 Mbps, a time spreading factor (TSF) is “2.” Thus, one symbol is transmitted four times, including a conjugate symbol.
Table 2 below shows an example of transmitting a symbol in an MB-OFDM system having a transmission rate ranging from 53.3 Mbps to 80 Mbps.
TABLE 2DataMapping DataD0C0D1C1. . .. . .D49C49D49*C50. . .. . .D1*C98D0*C99
Referring to Table 2, one data is transmitted two times, including a conjugate data. In other words, a transmitter transmits data D0 through D49 together with conjugate data D0* through D49*. In general, if the QPSK method is used, the transmitter divides one data into real and imaginary components and transmits the real and imaginary components.
The MB-OFDM system operating as described above is used in a method of modulating a UWB signal for fast short-distance communications. Federal Communications Commission (FCC) limits power emission for UWB communications to −41.3 dBm/MHz and limits levels to reduce interference among different frequency bands. According to IEEE 802.15.3a, power consumption is required to be 110 mW or less at a transmission rate of 110 Mbps, and 250 mW or less at a transmission rate of 200 Mbps.
Thus, an OFDM symbol must be transmitted at low power to use an MB-OFDM modulation method for UWB communications. Thus, technology for lowering a transmission power of an OFDM symbol is required. Also, low power UWB technology is required to apply UWB technology to mobile devices.